1. Field of the Invention
The present invention relates to an aqueous dispersion of fine resin particles used in such fields as printing materials for an electrophotographic toner, an ink, and the like, a coating material, an adhesive, a pressure-sensitive adhesive, fiber processing, paper manufacturing and paper processing, and civil engineering, and to a method for producing the aqueous dispersion of fine resin particles. In addition, the present invention relates to a method for producing toner particles using the aqueous dispersion of fine resin particles.
2. Related Background Art
Fine particles with a nonspherical form having a large ratio of a major axis to a minor axis are used as a material expecting potential improvements in various characteristics such as optical properties like light-scattering and light-focusing, friction properties such as sliding, adhesiveness, adherence, material mechanical properties such as impact resistance and tensile strength of an article, matting of a coating material, and shielding properties, in various fields such as an electronic and electric material, an optical material, a printing material, a biological and a medical material, and cosmetics, as a filler and a testing material. Many of widely used fine particles having a large ratio of a major axis to a minor axis are generally formed of inorganic materials such as a metal oxide. Such inorganic materials have larger specific gravity as compared with a resin and are not comfortable with a resin, and thus fine resin particles with a nonspherical form having a large ratio of a major axis to a minor axis are being desired.
To obtain an aqueous dispersion of fine resin particles, a suspension polymerization method, an emulsion polymerization method, a phase-inversion (self) emulsification method, and the like are know as the general methods. However, all of fine particles obtained by these methods are of a spherical form substantially. Fine particles with a high sphericity have drawbacks of too-high fluidity and a too-small surface area, leading to restriction in the processability in processing such as aggregating these fine particles and adhering them to other materials. Accordingly, fine resin particles with a nonspherical form having a large ratio of a major axis to a minor axis are being desired.
Fine resin particles having a large ratio of a major axis to a minor axis may also be produced by a mechanical method involving, for example, melting, spinning, and cutting. In this method, however, to make a particle size in an order of microns is not only technically difficult but also requires time and labor for mass production.
Resin particles having specific forms such as a circular disk and a planular plate, not like amorphous or spherical particles obtained by a pulverization method, a solution polymerization method, and the like, have been developed (refer to, for example, one of Japanese Patent Publication No. H06-53805 and Japanese Patent Application Laid-Open No. H05-317688). However, since these fine particles are in the form of plate, there is a steric limitation too, when they are processed to aggregate by themselves, to adhere to other substances, and the like.
Oval spherical particles of an organic polymer produced by a solution polymerization method are also proposed (refer to, for example, Japanese Patent Application Laid-Open No. 2007-70372), but no particles with a size of 1.0 μm or less is exemplified.
Further, a method for producing an aqueous dispersion of nonspherical fine resin particles by secondary aggregation of fine resin particles obtained by a molten emulsification method is also proposed (refer to, for example, Japanese Patent Application Laid-Open No. H08-269310), but making the ratio of a major axis to a minor axis beyond a certain level is difficult.
Therefore, so far fine resin particles expecting potential improvements in various characteristics as well as having a large ratio of a major axis to a minor axis and a size of sub-microns have not been known yet.
On the other hand, in the field of electrophotographic toner, to provide a production method with a low energy consumption and environmental burden and to supply such goods became a significant social requirement in an industrial sector amid the recent global movement to save energy. In addition, with current rapid diffuse of digitalization technology, needs for a higher quality picture in outputs such as printing and copying are being increased among users each day in an ordinary household, an office, and a publishing sector. In order to meet the need for a higher quality picture, improvement of a resolution power by making a particle diameter of toner smaller is one important technical approach especially in toners used for an electrophotographic picture. Presently, the weight-average particle diameter of toner particles can be made small in the order of 5 μm. However, in order to produce toners having the weight-average particle diameter of 6 μm or less while controlling the particle size distribution well, it is difficult to address the issues of production energy and cost by a conventionally used kneading-pulverization method. Accordingly, so-called chemical production methods, which can easily control a particle size distribution and a particle diameter of toners, such as a suspension polymerization method, a dissolution suspension method, and an emulsion aggregation method have also been used presently as the toner-production methods. Among them, an emulsion aggregation method draws an attention because the method can control a form and a dispersibility of particles intentionally.
One method for producing an aqueous dispersion of fine resin particles used in an emulsion aggregation method and the like is an emulsion polymerization method. In the emulsion polymerization method, a monomer is dispersed in water or a poor solvent to form an O/W emulsion, and a group of the dispersed monomer particles is polymerized by a radical polymerization to form an aqueous dispersion of fine resin particles. Accordingly, the emulsion polymerization method has been a method for producing an aqueous dispersion of fine resin particles applicable only to monomers which are polymerizable by a radical polymerization (for example, styrenic monomers, acrylic monomers and vinyl monomers). For this reason, an aqueous dispersion of fine resin particles obtainable by the emulsion polymerization has been limited to certain resins.
A dispersion granulation method may be cited as another method for producing an aqueous dispersion of fine resin particles. For example, a phase inversion emulsification method is one of such dispersion granulation methods. Specifically, a method in which a resin solution formed of a polyester resin having a neutralized salt structure dissolved in a water-miscible organic solvent is added by an aqueous medium to cause a phase-inversion emulsification and then the organic solvent is removed is known (refer to, for example, Japanese Patent Publication No. S61-58092 and Japanese Patent Publication No. S64-10547). However, in this phase inversion emulsification method, it is difficult to remove the organic solvent used in the production completely from the aqueous dispersion of fine resin particles. Even if the removal could be done, it may not only lead to a complicated production process and a high production cost but also risk to form fine resin particles with inhomogeneous sizes.
Further, a method for producing an aqueous dispersion of a polyester resin without using an organic solvent is known. For example, an aqueous dispersion of fine resin particles is obtained by a method in which a self-emulsifying thermoplastic resin is pressurized in an aqueous alkaline solution and heated at a temperature higher than a melting point of the resin for emulsification (refer to, for example, Japanese Patent Application Laid-Open No. H08-245769, Japanese Patent Application Laid-Open No. 2001-305796, Japanese Patent Application Laid-Open No. 2002-82485, and Japanese Patent Application Laid-Open No. 2004-287149). However, in this method for producing an aqueous dispersion of a polyester resin without using an organic solvent, a self-emulsifying resin like a specific polyester resin containing a sulfone group must be used. In addition, since the self-emulsifying resins have many dissociable terminal groups in themselves, when used, for example, as an electrophotographic toner, there is a risk of decreasing in its hydrophobicity and thereby decreasing such properties as an anti-static property and a water-adsorption.
In addition, a method for producing an aqueous dispersion of fine resin particles in which a resin melted at high temperature is mixed with an aqueous medium containing a neutralizing agent under high pressure and then the resulting mixture is applied with a shear force is known (refer to, for example, Japanese Patent Application Laid-Open No. 2000-191892 and Japanese Patent Application Laid-Open No. 2002-256077). However, in this method for producing an aqueous dispersion of fine resin particles by applying a shear force, a dispersant such as a surfactant is not contained basically, and thus a protection power (stereoscopic masking power) of fine resin particles thereby formed is weak. Accordingly, emulsification under pressure with heating is prone to cause coalescence among particles, and thereby likely to cause problems of forming undesired particle diameters and a broad particle size distribution easily.